The present invention relates to an image readout apparatus, and more particularly to an image readout apparatus for scanning a stimulable phosphor sheet storing a radiation image thereon with stimulating light to enable the stimulable phosphor sheet to emit light and for reading out the emitted light with a photoelectric conversion means, the image readout apparatus having an optical system isolated from undesirable vibration and arranged to prevent entry of light which would be responsible for noise.
There have recently been put to use systems for recording and reproducing radiation image information of an object by employing a stimulable phosphor. The stimulable phosphor is a phosphor which, when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, stores a part of the energy of the radiation. When the phosphor exposed to the radiation is exposed to stimulating light such as visible light, the phosphor emits light in proportion to the stored energy of the radiation.
In the systems for recording and reproducing radiation images, the radiation image of an object such as a human body is stored in a sheet having a layer of stimulable phosphor (hereinafter referred to as a "stimulable phosphor sheet" or a "phosphor sheet"), and then the stimulable phosphor sheet is scanned with stimulating light such as a laser beam to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected and converted to produce an image signal that is employed to record the radiation image of the object on a recording medium such as a photosensitive material, or to display the radiation image as a visible image on a display unit such as a CRT.
The aforesaid radiation image recording and reproducing system is disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428 and Japanese Laid-Open Patent Publication No. 56-11395, for example.
The system of the type described above is practically advantageous in that it can record images in a much wider radiation exposure range than that of conventional radiographic systems using an X-ray film and an intesifying screen. More specifically, it is known that the light emitted by the stimulable phosphor sheet upon exposure to the stimulating light is proportional in a wide range to the amount of radiation to which the stimulable phosphor sheet has been exposed. Therefore, even when the amount of radiation to which the stimulable phosphor sheet is exposed varies to a large extent under varying imaging conditions, the readout gain can be set to a suitable level to enable the photoelectric conversion means to convert the emitted light to an electric signal for recording or displaying the radiation image. Therefore, the radiation image can be produced irrespective of variations in the amount of exposure to which the stimulable phosphor sheet has been exposed.
After the radiation image stored on the stimuable phosphor sheet has been converted to the electric signal, the electric signal can be processed appropriately for recording a visible image on the recording medium or displaying such a visible image on the display unit. The radiation image thus reproduced is better for diagnostic purpose.
In the radiation image recording and reproducing system, the stimulable phosphor sheet is not used for permanently storing the radiation image, but employed for temporarily holding the radiation image before the image is finally recorded or displayed. Therefore, the stimulable phosphor sheet may recyclically be used for better economy and convenience.
The radiation image recording and reproducing system for such recyclic use of the stimulable phosphor sheet comprises a radiation image recording apparatus for irradiating an object with a radiation and for recording an image thereof on a stimulable phosphor sheet, an image readout apparatus for exposing the stimulable phosphor storing the radiation image to stimulating light to enable the stimulable phosphor sheet to emit light and for photoelectrically detecting the emitted light to convert same to an electric signal, and an erasing apparatus for exposing the stimulable phosphor sheet to erasing light to eliminate any remaining radiation image from the stimulable phosphor sheet thereby to make the latter reusable. The recording apparatus, the readout apparatus, and the erasing apparatus, and the eraser may be assembled as a unitary structure in a single casing, or may be constructed as individual and separate units.
The radiation image readout apparatus has a drawback in that when the readout apparatus is subjected to undesirable vibration at the time of exposing the stimulable phosphor sheet to the stimulating light such as a laser beam, the optical axis of the laser beam scanning the stimulable phosphor sheet is displaced with respect to the stimulable phosphor sheet, with the result that a radiation image cannot be read out with high accuracy. Such undesirable vibration may arise from the operation of a conveyor belt which delivers the stimulable phosphor sheet in the apparatus, or may be produced when the readout apparatus is shocked inadvertently by the person being diagnosed or the operator.
In reading out the image from the stimulable phosphor sheet upon exposure thereof to the stimulating light, it is preferable that the light be emitted from the stimuable phosphor sheet solely by application of the stimulating light. If any stray light entered the readout apparatus, it would also be photoelectrically converted, and no accurate radiation image information of the object could be available.